When a Cr.sub.2 O.sub.3 powder compact was heated at high temperature in air, a dense sinterd body could not be obtained. Therefore, the compact has been sintered by adding a small amount of metal oxide such as TiO.sub.2, SiO.sub.2, Al.sub.2 O.sub.3 or MgO. These additives form a liquid phase in Cr.sub.2 O.sub.3 grain boundaries during heating and promote densification of the compact.
A chromium oxide refractory brick produced by this method containes more than 20% of porosity, and has some drawbacks in use in a glass melting furnace; it is subjected to corrosion by molten glass, and tends to generate pores in molten glass and to vaporize at high temperature.
Chromium has many oxidation states, 2.sup.+, 3.sup.+, 4.sup.+ and 6.sup.+ and readily changes at elevated temperatures from one to the other depending on the environment. In heating in an oxidizing atmosphere Cr.sub.2 O.sub.3 has a strong tendency to react with oxygen to form more volatile oxide such as CrO.sub.3. This explaines why a dense body cannot be obtained by sintering in air.
P. D. Ownby et al. have reported that Cr.sub.2 O.sub.3 powder compact could be sintered to almost theoretical density at 1600.degree. C. at PO.sub.2 =2.times.10.sup.-12 atm. which was controlled using a CO/CO.sub.2 buffer system. (P. D. Ownby and G. E. Jungquist, J.Amer.Ceram.Soc., 53 [9] 433-36 (1972).)
Since it is difficult to control on an industrial scale, the sintering atmosphere with CO and CO.sub.2 gases at high temperature, chromium oxide refractory brick produced using this atmosphere control method has not been utilized yet.
It is an object of the invention to provide an industrially simple method for producing a highly pure and fully dense Cr.sub.2 O.sub.3 body.
Another object of the invention is to provide a method for producing a Cr.sub.2 O.sub.3 body with desired porosity.
A further object of the invention is to provide an economical, crack-free and microstructurally uniform sintered body in an industrial scale.